In the production of a fluorinated copolymer solution, there is a problem such that due to an inorganic acid component such as hydrofluoric acid formed in the polymerization step, the fluorinated copolymer solution is likely to lose stability during or after the polymerization, whereby gelation is likely to proceed, or the molecular weight is likely to increase.
Further, during the storage of the obtained fluorinated copolymer solution, the gelation or the increase of the molecular weight sometimes occurred and led to such a problem that the subsequent processing was difficult, or the miscibility with other components was adversely influenced.
Patent Document 1 discloses that at the time of polymerization for a fluorinated copolymer, an alkali metal carbonate is added to the polymerization system to neutralize an acid component formed during the polymerization thereby to facilitate a smooth progress of the polymerization and to improve the storage stability for a long period of time, of the fluorinated copolymer solution obtained by the polymerization.
However, in the method disclosed in Patent Document 1, it is required to remove the alkali metal carbonate by e.g. filtration after the polymerization, thus leading to such a problem that the production steps tend to be complex and long. Further, even if the alkali metal carbonate is removed, there still remain such a problem that an alkali metal carbonate dissolved in a very small amount will precipitate, whereby the transparency of the fluorinated copolymer solution will be lost.
Further, Patent Document 2 proposes to prevent gelation during polymerization by conducting the polymerization for a fluoro-olefin copolymer in the presence of a compound containing a 2,2,6,6-tetra-substituted piperidyl group.
However, in the method disclosed in Patent Document 2, there was a problem such that the storage stability of the obtained fluoro-olefin copolymer solution for a long period of time was inadequate.
Further, in the method disclosed in Patent Document 2, in a case where the obtained fluoro-olefin polymer solution is to be used as a coating composition, the dryability of the coating film is inadequate.